This invention relates to an ion gun for use in an ion beam processing apparatus, to an ion beam processing apparatus incorporating same and to an ion beam neutraliser.
There have been various proposals in the prior art to use broad beam ion sources for surface modification of substrates, either by sputter deposition, sputter etching or milling. Such broad beam ion sources utilise multiaperture ion optics and typically range in diameter from about 25 mm up to about 500 mm.
In a typical ion beam source (or ion gun) a plasma is produced by admitting a gas or vapour to a low pressure discharge chamber containing a heated cathode and an anode which serves to remove electrons from the plasma and to give a surplus of positively charged ions which pass through a screen grid or grids into a target chamber which is pumped to a lower pressure than the discharge chamber. Ions are formed in the discharge chamber by electron impact ionisation and move within the body of the ion gun by random thermal motion. The plasma will thus exhibit positive plasma potential which is higher than the potential of any surface with which it comes into contact. Various arrangements of grids can be used, the potentials of which are individually controlled. In a multigrid system the first grid encountered by the ions is usually positively biased whilst the second grid is negatively biased. A further grid may be used to decelerate the ions emerging from the ion source so as to provide a collimated beam of ions having more or less uniform energy. Ion guns displaying high current operation and delivering ion energies in the range up to about 1500 volts, as a generic type find wide use in thin film technology. For ion sputtering a target is placed in the target chamber where this can be struck by the beam of ions, usually at an oblique angle, and the substrate on to which material is to be sputtered is placed in a position where sputtered material can impinge on it. When sputter etching or milling is to be practised the substrate is placed in the path of the ion beam.
Hence, in a typical ion gun an ion arriving at a multiaperture extraction grid assembly first meets a positively biased grid. Associated with the grid is a plasma sheath. Across this sheath is dropped the potential difference between the plasma and the grid. This accelerating potential will attract ions in the sheath region to the first grid. Any ion moving through an aperture in this first grid, and entering the space between the first, positively biased grid, and the second, negatively biased, grid is strongly accelerated in the intense electrical field. As the ion passes through the aperture in the second grid and is in flight to the earthed target it is moving through a decelerating field. The ion then arrives at an earthed target with an energy equal to the voltage of the first, positive, grid plus the sheath potential.
Although such prior art ion beam sources operate satisfactorily with inert gases the life of the cathode is severely limited if a reactive gas, such as oxygen, fluorine or chlorine, is added to or replaces the inert gas used to generate the plasma. For this reason it is unsatisfactory to use such ion beam sources with reactive gases.
A review of broad beam ion sources has appeared, viz: "Technology and applications of broad-beam ion sources used in sputtering. Part I. Ion Source technology" by H.R. Kaufman, J.J. Cuomo, and J.M.E. Harper, J. Vac. Sci. Technol, 21(3), Sept./Oct. 1982, pages 725 to 736. The second part of this review, viz: "Part II. Applications" by J.M.E. Harper, J.J. Cuomo, and H.R. Kaufman appeared in the same journal, immediately following Part I thereof, at pages 737 to 755. The same authors published a further paper "Developments in broad-beam, ion-source technology and applications" in the same issue of the same journal at pages 764 to 767. A more recent report from Harold R. Kaufman appeared under the title "Broad-beam ion sources Present status and further directions", J. Vac. Sci Technol A 4(3), May/June 1986 at pages 764 to 771.
The use of a capacitively coupled rf discharge plasma for generation of ion beams has been proposed by C. Lejeune et al in a paper "Rf multipolar plasma for broad and reactive ion beams", Vacuum, Vol. 36, Nos. 11/12 (1986), pages 837 to 840 and in EP-A-0200651.
Another form of r.f. broad beam ion source with magnetic confinement, using a multipole arrangement with magnets arranged with alternate north and south poles facing tne plasma, has been described by R. Lossy and J. Engemann in papers entitled "Rf-broad-beam ion source for reactive sputtering", Vacuum, 36, Nos. 11/12, pages 973 to 976 (1986) and "Characterization of a reactive broad beam radio frequency ion source", J. Vac. Sci. Technol. B6(1), Jan/Feb 1988, pages 284 to 287.
Plasma generation by means of r.f. excitation relies upon the ability of electrons to respond to the high frequency field and the inability of ions to do so because of their relatively high inertia. As a result electrons are stripped off the gas molecules. The electrons then become trapped by the magnetic confinement cusps formed by the alternately north and south poles which face the plasma, leaving a positively charged plasma in the central part of the plasma generation chamber Because prior art designs involving r.f. capacitive excitation produce high plasma potentials of the order 200 V to 300 V the ions are accelerated very fast towards the first grid and hit this with high energy thereby heating it and tending to cause sputtering of grid material which in turn can give rise to contamination in the extracted ion beam.
Another design of ion beam source in which an inductively coupled r.f. power source is used to excite a gaseous material to a plasma state and to produce atomic ions rather than molecular ions is described in GB-A-2162365. In this design a quartz chamber in the shape of a bell jar is surrounded by a coil which is connected both to a radio frequency power source and also to a second power source which is isolated by means of coils from the radio frequency power source and which is adapted to provide a steady solenoidal magnetic field in the region of the major part of the wall of the chamber. A metal plate closes the otherwise open end of the bell jar except for an exit hole for ions produced by the source and acts as an extraction electrode. In use the wall of the chamber can reach a temperature of about 600.degree. C. Although this ion source operates reasonably satisfactorily at frequencies of about 2 MHz, it is not practicable to operate at higher, more commercially desirable frequencies, such as 13.56 MHz or a multiple thereof.
Another form of ion source with a solenoidal coil surrounding the chamber in which a plasma is to be generated is described in GB-A-2180686.
Although extracted ion beams from all ion sources are naturally space-charge neutralised, current neutralisation at an insulating target is not, however, ensured. Hence in ion beam processing it is desirable to provide a surplus of moderately accelerated electrons, apart from the thermal electrons that will always be present so as to provide the necessary electron flux to an insulated target to afford current neutralisation. In this way the development of a charge on the target or substrate is avoided. A number of designs of neutraliser have been proposed. A typical neutraliser uses a hollow cathode discharge, fed either with mercury vapour (for space propulsion applications) or with argon (for other applications), to form a plasma bridge that acts as an electron source. This plasma bridge effuses out of the hollow cathode through a small diameter aperture bored in a hot tungsten tip that typically operates at about 1000.degree. C. Another design of neutraliser, that also uses a hot cathode to generate a plasma with argon as the plasma forming gas, has been proposed in a paper entitled "Electrostatic reflex plasma source as a plasma bridge neutralizer" by C. Lejeune, J.P. Grandchamp and O. Kessi, Vacuum, Volume 36, Nos. 11/12, pages 857 to 860 (1986). Further description of the use of neutralisers will be found in the other papers cited above.
A drawback to use of the conventional designs of neutraliser is that, as they employ hot cathodes to generate a plasma, it is necessary to utilise an inert gas or vapour from which to generate the plasma. For certain applications involving use of a reactive gas the introduction of an inert plasma forming gas, such as argon, may be disadvantageous. However, if a neutraliser with a hot cathode is supplied with a reactive gas, this will soon result in destruction of the cathode.
In the r.f. excited ion guns of the prior art a plasma is generated in the magnetic confinement chamber. For optimum operation of the ion gun it is important that the plasma from which the ions are accelerated shall be of high density, as uniform as possible, and at as low a potential as possible. However, these aims cannot be satisfactorily met with the prior art designs.